1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser device and a laser module, and more particularly, to a gain coupling type or complex coupling type distributed feedback semiconductor laser device and a laser module which is provided with the distributed feedback semiconductor laser device.
2. Description of the Art
A distributed feedback semiconductor laser device (hereinafter referred to as xe2x80x9cDFB laser devicexe2x80x9d) is designed in such a way that a diffraction grating for periodically changing the real refractive index (real part) or the extinction coefficient (imaginary part) of a complex refractive index is formed inside a resonator which has a predetermined lamination structure of semiconductor materials and a predetermined resonator length to provide such a wavelength selectivity as to allow feedback to be applied only to a laser beam of a specific wavelength.
The complex refractive index N is generally expressed by
N=nxe2x88x92ik 
where n is the real refractive index, k is the extinction coefficient, and i is an imaginary unit.
Such DFB laser devices are generally classified into three groups depending on the type of the diffraction grating. The groups are (1) a refractive index coupling type which has such a structure that only the real refractive index (real part) n of the complex refractive index N changes periodically, (2) a gain coupling type which has such a structure that only the extinction coefficient (imaginary part) k of the complex refractive index N changes periodically, resulting in a periodic change only in gain, and (3) a complex coupling type which has such a structure that both the real refractive index n and the extinction coefficient k change periodically.
Such DFB laser devices are used widely in optical communication applications, and demanded of an existence of one longitudinal mode laser light with maximum gain (hereinafter referred to as xe2x80x9ca high single-mode yieldxe2x80x9d) and a high slope efficiency as well as a high resistance to slight reflected return lights from outside.
The conventional DFB laser devices, however, do not have a sufficient resistance to the reflected return lights from outside. In a trunk line system, which requires a stable operation, therefore, an optical isolator needs to be incorporated in a DFB laser module in which a DFB laser device is installed. In a subscriber""s line system, it is also essential to incorporate an optical isolator in a DFB laser module in case where the DFB laser module is for a fast usage.
However, the DFB laser module that is used in a subscriber""s line system strongly requires a low cost. Therefore, in the case where a DFB laser module, which is used for at least a relatively low speed usage, required is one which usage need not incorporate an optical isolator.
In case of manufacturing such a DFB laser module that does not have a built-in optical isolator, it is very important that as little external reflected and returned light as possible be allowed to enter the DFB laser device. To reduce the reflected and returned light, therefore, normally AR (Anti-Reflection) coating is applied to the surface of the lens that optically couples the DFB laser device to the optical fiber, or the facet of the optical fiber is subjected to skew polishing and AR coating is applied to the polished facet.
To increase the optical output of a DFB laser device itself and achieve a stable single-mode oscillation to thereby acquire a high single-mode yield, normally, AR coating is applied to that facet of the DFB laser device from which a laser beam is output (hereinafter this output facet is called xe2x80x9cfront facetxe2x80x9d or xe2x80x9cfirst facetxe2x80x9d while the facet opposite to the front facet is called xe2x80x9crear facetxe2x80x9d or xe2x80x9csecond facetxe2x80x9d). The AR coating applied to the front facet, however, reduces the resistance to slight reflected and returned light, which brings about an unstable operation.
In other words, reducing the reflectance of the front facet of a DFB laser device lowers the resistance to slight reflected and returned light while improving the single-mode yield and the slope efficiency (the differential efficiency of the current vs. optical output characteristic in an oscillation state). Increasing the reflectance of the front facet, on the other hand, reduces the single-mode yield and slope efficiency while increasing the resistance to slight reflected and returned light. Apparently, the resistance to slight reflected and returned light has a trade-off relation with the single-mode yield and the slope efficiency, making it difficult to achieve both at the same time.
Accordingly, it is an object of the present invention to provide a DFB laser device and a laser module, which can stably operate in a single mode oscillation over a wide temperature range without using an optical isolator.
A distributed feedback semiconductor laser device according to the present invention, which has a resonator for oscillating a laser beam, comprises a diffraction grating, formed inside the resonator, for periodically changing only an extinction coefficient k (gain coupling type) or both a real refractive index n and the extinction coefficient k (complex coupling type) in a complex refractive index N (expressed by N=nxe2x88x92ik where i is an imaginary unit), the resonator having a front facet having a first reflectance; and a rear facet opposite to the front facet and having a second reflectance, the first reflectance being smaller than the second reflectance and equal to or larger than 10%, preferably equal to or smaller than 20%.
According to the gain coupling type or complex coupling type distributed feedback semiconductor laser device of the present invention, as the reflectance of the front facet is made lower than the reflectance of the rear facet and set equal to or larger than 10%, it is possible to realize a stable single-mode operation over a wide temperature range and ensure a high single-mode yield and a high slope efficiency as well as a high resistance to slight reflected return lights.
A laser module according to the present invention comprises a distributed feedback semiconductor laser device having a resonator for oscillating a laser beam, the semiconductor laser device comprising a diffraction grating, formed inside the resonator, for periodically changing only an extinction coefficient k or both a real refractive index n and the extinction coefficient k in a complex refractive index N expressed by N=nxe2x88x92ik where i is an imaginary unit, the resonator having a front facet having a first reflectance and a rear facet opposite to the front facet and having a second reflectance, the first reflectance being smaller than the second reflectance and equal to or larger than 10%, preferably equal to or smaller than 20%; an optical fiber which has a fiber facet and transfers a laser beam emitted by the semiconductor laser device; and a lens for optically coupling the semiconductor laser device to the optical fiber to cause the laser beam emitted by the semiconductor laser device to enter the fiber facet.
As the DFB laser device according to the present invention is installed in the laser module, it makes it unnecessary to incorporate an optical isolator and it is possible to provide a low-cost laser module which is used in, for example, a subscriber""s line system.
According to a preferable mode of the distributed feedback semiconductor laser device of the present invention, a resonator for oscillating a laser beam comprises a first cladding layer, an active layer deposited on the first cladding layer, a second cladding layer deposited on the active layer and an electrode for injecting a current into the active layer via the second cladding layer.
The diffraction grating is preferably an absorption diffraction grating which is formed in the second cladding layer in a vicinity of the active layer and has a plurality of absorption portions arranged in a given periodicity.
It is also preferable that the diffraction grating be an active-layer etching diffraction grating and the active layer and the second cladding layer have projections and recesses alternately formed in a given periodicity at their interface so that the projections and recesses of the active layer respectively engage with the projections and recesses of the second cladding layer.
It is further preferable that the diffraction grating be a current blocking diffraction grating which is formed in the second cladding layer in a vicinity of the active layer and has a plurality of current blocks arranged in a given periodicity to suppress the current injected into the active layer from the electrode.